When producing hardcopy images using a laser printer, a latent image is first created on the surface of an insulating, photo-conducting material. This photo-conducting material is usually formed into a rotating drum. The insulating, photo-conducting material is made conductive when and where it is exposed to light.
Data, which defines the image to be printed, is used to modulate a laser that is scanned over the surface of the drum, line-by-line. A field of charges is applied over the surface of the drum. By selectively exposing areas on the surface of the drum with the laser, the charge in the exposed area is dissipated. This creates the latent image in the charge field on the drum that corresponds to the image represented by the data used to modulate the laser.
The latent image on the drum is then developed. A charged toner is applied to the surface of the drum. Because the toner is charged, the toner will be attracted to the latent image on the drum and repelled by other, unexposed portions of the drum surface. Thus, the latent image on the drum is developed and becomes a toner image on the drum surface.
The toner image is then transferred from the drum to a print medium, such as a sheet of paper. The toner is then fixed or fused to the print medium, typically with heat. The result is a hardcopy document bearing the image that corresponds to the data used to modulate the laser.
As used herein, the term “print engine” includes the devices used to actually produce a desired hardcopy document. Thus, a laser print engine includes, for example, a laser, a photoconductive drum, a transfer roller, laser modulation circuitry, the image data processing circuitry, etc. The speed at which a laser printer may print is limited mostly by the characteristics and physical mechanics of the print engine, i.e., the processing speed of the circuitry and the motion of the mechanical parts.
Within the data used to define the image being printed, the image is broken down into pixels. Each pixel is a small portion of the image. The pixels are arranged in successive lines to form the image. In monochromatic laser printing, each pixel is associated with a particular darkness or brightness along a grayscale. If a pixel is to be completely dark, then a maximum amount of black toner should be applied to that pixel during printing. Conversely, if the pixel is to be completely light, then no toner should be applied. In between these two extremes, varying amounts of toner are applied to produce various shades of gray.
The amount of toner applied within a pixel will be determined by how much of that pixel's area on the photo-conducting drum is exposed by the laser. By placing toner in only a varying portion of a pixel region, it is possible to create the effect of various shades of gray for each pixel and improve the resolution of the resulting image. Consequently, the laser can be pulsed for a selective amount of time within each pixel as it is scanned across each line of the image. The method of selectively pulsing the laser as described above is referred to as pulse width modulation (PWM).
To coordinate operations, complex devices, such as a laser printer, use clock signals. A laser printer can use two separate clock signals, a system clock and a video clock. The system clock regulates the operation of the data processing elements of the printer, e.g., the central processing unit (CPU) and memory. The system clock runs at a speed defined by processor performance. The video clock regulates the transfer of video data in synchronization with the operation of the print engine. The video clock may be a fixed-frequency crystal oscillator with a frequency that is selected to match the performance speed of the print engine. In other words, the period of the video clock is influenced by the physical mechanics, characteristics and limitations of the elements of the print engine. By choosing an appropriate video clock frequency, the transfer of video data is synchronized with the operation of the print engine.
However, using two unrelated clocks can cause other issues. First, delays may occur when data is transferred from the processor to the video circuitry of the print engine. Second, ASIC (Application Specific Integrated Circuit) design and testing for use in a printer is significantly hampered due to the difficulty of communicating using two different clock domains. Third, ASIC's designed for one print engine may have difficulty functioning with another print engine, thereby limiting the ability to reuse the ASIC in future laser printer development. Implementations that make use of a single clock, such as the system clock, for performing the functions of regulating the operation of the data processing elements of the printer and regulating the transfer of video data in synchronization with the operation of the print engine can encounter difficulties in dividing down the system clock for use by the video pixel generation circuitry.